GB2138417A

GB2138417A - A method of enamelling an article

Info

Publication number: GB2138417A
Application number: GB08334010A
Authority: GB
Inventors: Jean-Pierre DEVILLE
Original assignee: SEB SA
Current assignee: SEB SA
Priority date: 1983-04-15
Filing date: 1983-12-21
Publication date: 1984-10-24
Also published as: GB2138417B; GB8334010D0

Abstract

The method comprises applying a first enamel slip coating, heating to effect partial evaporation of water, applying a second enamel slip whilst the first coating is above ambient temperature, heating to evaporate the volatile components and then firing. The coatings (3, 4) may be applied to the bottom face (1) of an aluminium or aluminium alloy culinary vessel. <IMAGE>

Description

SPECIFICATION A method of enamelling on article and on enamelled article whenever produced thereby THIS INVENTION relates to a method of enamelling articles, as well as to articles whenever produced thereby. The method con be used to enamel the bottom of a culinary vessel such as a saucepan, frying pan, deep frying pan or the like of aluminium or aluminium alloy.

Enamels which have previously been used for enamelling aluminium or aluminium alloys suffer from the disadvantage that they are soft ond afford low resistance to abrasion and scoring. This disadvantage results from the particular composition of the enamel frit employed, the sintering temperature of which must necessarily be lower than the melting point of the aluminium or aluminium alloy being enamelled.

It has been proposed to harden these enamels by adding hardening agents, as described in French Patent Specification No.

2,289,627. However, the addition of such hardening agents is attended by major drawback in that it has the effect of considerably reducing the adhesiveness of the enamel coating.

Because of the above disadvantages, it has been usual during manuf acture to form an enamel coating only on the side wall of the utensil whilst the metal of the bottom face remains uncoated. At the time of manufacture it is convenient to coat the entire external surface of the utensil with enamel then to remove the enamel from the bottom of the utensil by etching. Apart from the fact that the bottom of the utensil is consequently difficult to clean, this etching operation reduces the thickness of the metal of the bottom wall, so that it is necessary to use a greater thickness of metal for making the utensil than would otherwise be necessary in order to obtain sufficient mechanical strength. This has the effect of increasing the cost of manufacture.

It is an object of the present invention to enable the provision of method of enamelling articles whereby the above disadvantages may be overcome or at least mitigated.

According to a first aspect of the present invention there is provided a method of enamelling an article, which method comprises applying a first enamel slip coating to a surface of the article, exposing the first enamels coating to sufficient heat partialy to evaporate water therefrom, applying a second enamel slip coating to the first enamel slip coating whilst the first enamel slip coating is above ambient temperature, heating the second enamel slip coating so as completely to evaporate the volatile components thereof and firing the first and second enamel slip coatings, preferably immediately,in order to sinter the enamels.

The method may,for example, be a method of enamelling the bottom face of a culinary vessel of aluminium or aluminium alloy, wherein the second enamel slip coating in parts abrasion resistance.

The invention further provides an enamelled article whenever produced using the method.

The practical applcation of the method is extremely rapid, in particular because it dispenses with the need for intermediate firing of the first enamel coating. Apart from the fact that the high speed of execution of the method enables considerable savings in time and energy, the aluminium alloy is heated to the enamel firing temperature for a relatively short period of time in the course of a single firing operation, with the result that the mechanical properties thereof are not liable to be in paired by unduly long exposure at this temperature.

Furthermore, it has been found that surprising advantages are obtained by applying the second coating of enamel on a first enamel layer which has not been fired but has been partialy evaporated and is stil hot. When this procedure is adopted, the first enamel coating in the form of a hot and porous "biscuit" exhibits with respect to the second enamel coating absorbent properties such that the com ponents of the second coating infitrate into the first coating, with the result that exceptional adhesion between the two coatingsis obtained.

The step of applying the second enam el coating to a first coating of unfired enam el is contrary to conventional wisdom in the art, because it would be expected that such a procedure would give rise to a first coating having insufficient m echanical cohesion to withstand the subsequent operations.

Moreover, excellent adhesion is obtained between the first enamel coating and the surface of the cooking vessel since the coating com position employed for this purpose can be any composition, for example a conventional composition, which is endowed with this property.

Furthermore, the method of the present invention permits the use of a second enamel coating having a composition which is different from that of the first coating, thereby making it possible to obtain excelent resistance to abrasion and to scoring while at the same time adhering very strongly to the first coating as explained above whereas the second coating would not offer sufficient adhesion to the surface of the culinary vessel if it were appled directly thereto.

The present invention thus enables the provision of a method of enamelling the bottom of a culinary vessel which is particularly econom ical to perform and makes possible the production of an enamel which adheres stron gly to the bottom face so that the bottom is easy to clean, the enam el also being abrasion resistant.

In an advantageous embodiment of the method, the first layer of enamel slip is heated to a temperature of 50 to 95"C during a period of time sufficient to reduce the moisture content to a value of 0.5 to 10% by weight.

With such a low moisture content, the layer of particles of enamel frit appears in the form of a "biscuit" having a sufficient degree of mechanical cohesion for subsequent handling operations while having a high absorption capacity for the enamel slip which is appled subsequently.

Preferably, the second enamel coating is applied to the first enam el coating before the temperature of the latter has fallen to a value below about 45"C.

Preventing complete coolng of the first coating permits energy saving during the subsequent complete evaporation step and improves the adhesion of the second enamel coating.

In a preferred embodiment of the invention, the second enamel coating is applied by the screen deposition process. Experience has in fact shown that the cohesion of the first coating is wholly compatible with the applcation of a deposition screen thereto.

According to a second aspect of the invention, a culinary vessel of aluminium or aluminium alloy having an enamelled bottom wall and obtainable for example by means of the method of the present invention is provided with two superimposed coatings of enamel sintered at a temperature which is compatible with the melting point of the aluminium or aluminium alloy. The outer coating contains a suitable charge of hardening agents for making the coating resistant to abrasion and the inner coating contains a smaller charge of hardening agents in order to permit optimum adhesion between the inner coating and the bottom of the vessel.

It has been found that, when the first enamel coating contains less than about 10% of hardening agents,the presence of the latter do not affect the adhesion of the first coating to the surface of the utensil.

On the other hand, the presence of such hardening agents in a proportion in the range of 1 5 to 40% by weight in the second coating endows the latter with very high abrasion resistance.

An enamelled vessel in accordance with the invention is thus econom ical to produce and very easy to clean.

For a better understanding of the present invention, and to show how the same may be put into effect, reference wil now be made, by way of example,to the accompanying drawing,in which: Figure 1 is a partial side sectional view of the bottom wall of an enamelled culinary vessel in accordance with the invention, and Figure 2 is a partial bottom plan view of the vessel of Figure 1.

A culnary vessel I such as a frying pan of alum inium or aluminium alloy is provided on the bottom wall 2 thereof with two superimposed enamel layers or coatings 3, 4 obtained from an enamel frit which is sinterable at a temperature below the melting point of the aluminium or aluminium alloy.

The outer coating 4 contains a suitable charge of hardening agents for endowing the coating with abrasion resistance. The inner coating 3 which is adjacent the bottom wall 2 contains a smaller charge of hardening agents in order to permit optimum adhesion between the inner coating and the vessel bottom wall 2.

The enamel coatings 3 and 4 are obtained from an enamel frit of conventional type for aluminium or aluminium alloys, to which is added hardening agents such as a-alumina, silicon carbide, quartz, tungsten carbide, silcoaluminates or metals such as tungsten, titanium and stainless steel in the form of fine particles.

It has been found that excellent abrasion resistance is obtained when the outer coating 4 contains between 1 5 and 40% by weight of hardening agents.

It has also been found that, in order to obtain excellent adhesion of the coating 3 to the bottom face 2 of the vessel, the coating material should preferably contain less than 10% by weight of hardening agents.

As can clearly be seen in the accompanying drawing, the outer enamel layer 4 which is hardened by means of the hardening agents comprises a series of concentric rings 4a which are raised with respect to the first layer 3,the rings being separated from each other by grooves 5.

The intended function of these projecting rings 4a is to reduce the contact surface between the bottom of the utensil and the heating surface or other kitchen work surface in order to limit the wear of the enam el layer 4 solely to the projecting rings.

Another function of the projecting rings 4a is to provide an attractive appearance.

The projecting rings 4a could be replaced by spirals, a square mesh pattern, a checkerboard pattern, a raised-button pattern or inscriptions in relief.

The method of applcation of the enamel coatings 3 and 4 to the bottom or underface 2 of the culinary vessel will now be described.

The first step comprises compressed-air spraying on the underface 2 of the aluminium vessel a slip of enamel frit of a conventional type for aluminium , to which there have been added for example 10% of one or more hardening agents such as fine particles of silicon carbide.

The coating is then heated to a temperature of 50 to 95"C, and preferably 70"C, during a period of time sufficient to ensure that the residual moisture content of the sip coating is 0.5 to 10% by weight.

The coating then consists of a "biscuit" having a sufficient degree of cohesion to guard against destruction during subsequent handling of the vessel Before the temperature of the "biscuit" has fallen below approximately 45C, an enamel paste is applied by a screen deposition process to the "biscuit" while the latter is stil hot. The enamel paste contains a conventional enamel frit for aluminium , mineral pigments, pine oil as a liquid vehicle, alcohol and so on. Hardening agents such as slicon carbide are added to the enamel paste in a proportion which is preferably 20% by weight but may vary between 1 5 and 40%.

In the screen deposition process,the paste is appled on a mask which serves to produce a relief pattern on the innerlayer of enamel Because the "biscuit" of enamel particles of the first coating has been reduced virtually to a zero moisture content, the porosity of the biscuit is such as to enable it to absorb the enamel paste appled thereto by screen deposition. Also, because the biscuit is still hot at the time of application, the viscosity of the oil which serves as a vehicle for the screen deposition paste decreases, so that the oil infiltrates into the pores of the biscuit and thus entrains particles of the enamel paste into the pores.

Substantially complete adhesion is thus obtained between the outer layer of enamel applied by the screen deposition process and the first layer which contains a relatively low proportion of hardening agents.

The coatings of enamel particles applied to the vessel are then reheated in order to achieve complete removal of the volatile components of the coatings.

The vessel is then introduced into a furnace in order to fire the coatings of enamel frit so as to sinter the same and convert them to enam el. The firing temperature is about 560C in the example considered.

Thus, there is obtained a culinary vessel having a bottom face 2 coated with a first layer of enamel 3 which has a thickness of about 50 microns and which adheres strongly to the metal and with a second layer of enamel 4 having a thickness of about 20 microns and high abrasion resistance because of the presence of a large proportion of hardening agent(s).

The second layer of enamel 4 adheres strongly to the first layer 3 as a result of partial interpenetration of the two layers which is obtained using the method described above. The excellent adhesion of the two enamel layers 3 and 4 can be demonstrated by subjecting the culinary vessel to repeated thermal shocks.

By way of example, the basic composition of the enamel frit employed for each of the layers or coatings 3 and 4 can be as follows: B203: 3 parts; KOH : 2 parts; ZnO:10 parts ; Six2: 6 parts the rem ainder comprising the liquid vehicle.

The method described above is particularly rapid and economical because it dispenses with the need for intermediate firing of the first enamel coating 3. As a consequence, the aluminium of the vessel is no longer subjected for an excessive length of time to a high temerature which would be liable to impair its mechanical properties and to produce deformation of the vessel thus resulting in poor thermal contact between the bottom of the vessel and the heating surface.

Also, enamelling of the underface 2 of the vessel makes it un-necessary to perform conventional etching of the underface, with the result that the thickness of the metal of the bottom wall is unaffected. It is thus possible to produce a culinary vessel in which the thickness of the metal of the entire vessel can be smaller than the thickness which has been previously adopted,thereby reducing the cost of manufacture.

In alternative methods in accordance with the invention, the second enamel coating 4 can be applied to the first coating 3 by a method other than the screen deposition process. The second coating 4 can for example be applied by compressed-air spraying in the same manner as the first coating, by decalcomania or by spray-gun projection through a mask or the like. As will readly be understood,the first enamel coating 3 can cover the entire external and internal surface of the vessel The invention can be applied to any enamel composition for aluminium articles,such com positions being well-known in the art generally having a sintering temperature below about 600"C.

Claims

1. A method of enamelling an article, which method comprises applying a first enamel slip coating to a surface of the article, exposing the first enamel slip coating to sufficient heat partially to evaporate water there-ffrom , applying a second enamel slip coating to the first enamel slip coating whilst the first enamel slip coating is above ambient temperature, heating the second enamel slip coating so as completely to evaporate the volatile components thereof and firing the first and second enamel slip coatings.

2. A method according to Claim 1, which is a method of enamelling the bottom face of a culinary vessel of 'aluminium or aluminium alloy and wherein the second enamel slip coating in parts abrasion resistance.

3. A method according to Claim 1 or 2, wherein the first enamel slip coating is heated to a temperature of 50 to 95"C for a period of time sufficient to reduce the moisture content thereof to 0.5 to 10% by weight.

4. A method according to any one of the preceding claims, wherein the second enamel slip coating is applied to the first enamel slip coating before the temperature of the first enamel slip coating has fallen below about 45"C.

5. A method according to any one of the preceding claims, wherein the second enamel slip coating is applied by screen deposition.

6. A method according to any one of the preceding claims, wherein the enamel slip which is used for forming at least the second coating contains at least one hardening agent.

7. A method according to Claim 6, wherein the enamel slip which is used for forming the second coating contains a higher proportion of hardening agent(s)than that used for forming the first coating.

8. An enamelled article whenever produced using the method of any one of the preceding claims.

9. A method of enamelling an article, which method comprises applying to a surface of the article two superimposed enamel coatings sinterable at a temperature which is compatible with the melting point of the material comprising the surface, wherein the outer coating comprises one or more hardening agents for increasing the abrasion resistance thereof and the inner coating comprises no hardening agent(s) or a smaller proportion of hardening agent(s) than the outer coating, to faclitate adhesion between the inner coating and the said surface.

10. A culinary vessel of aluminium or aluminium alloy having an enam ell ed bottom face, wherein the vessel is provided with two superimposed coatings of enamel sinterable at a temperature which is compatible with the melting point of the aluminium or aluminium alloy, the outer coating contains a charge of hardening agents for making the said outer coating resistant to abrasion and the inner coating contains a smaller charge of hardening agents, to permit good adhesion between the inner coating and the bottom face of the vessel

11. A vessel according to Claim 10, wherein the outer coating contains a proportion of hardening agent(s) in the range of from 1 5 to 40% by weight and the inner coating contains a proportion of hardening agent(s) in the range of from 0 to 10 by weight.

12. A vessel according to Claim 10 or 11, w herein the hardening agent(s) is/are selected from alumina, silcon carbide, quartz, tungsten carbide, the silcolalum inates and the hard metals in the form of fine particles which are uniformly distributed within the coating(s)

1 3. A vessel according to any one of Claims 10 to 1 2 wherein adhesion between the inner and outer coatings is enhanced by partial interpenetration thereof.

14. A method of enamelling a culinary vessel substantialy as herein-before described with reference to the accompanying drawing.

1 5. A culinary vessel substantially as hereinbefore described with reference to,and as shown in,the accompanying drawings.

1 6. Any novel feature or combination of features described herein.